The invention relates generally to fuel cell stacks and modules for power generation, and more particularly, to the sealing of solid oxide fuel cell stacks.
Fuel cells, for example solid oxide fuel cells (SOFCs), are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. The fuel cell operating temperatures depend on the material forming the ionic conducting layer. Desirably, power generation systems incorporating high-temperature fuel cells have the potential for higher efficiencies and power outputs. Exemplary high-temperature fuel cells have operating temperatures above about 600° C., and SOFCs typically operate in a range between about 800° C.-1000° C.
A fuel cell produces electricity by catalyzing fuel and oxidant into ionized atomic hydrogen and oxygen at, respectively, the anode and cathode. The electrons removed from hydrogen in the ionization process at the anode are conducted to the cathode where they ionize the oxygen. In the case of a solid oxide fuel cell, the oxygen ions are conducted through the electrolyte where they combine with ionized hydrogen to form water as a waste product and complete the process. The electrolyte is otherwise impermeable to both fuel and oxidant, and merely conducts oxygen ions. This series of electrochemical reactions is the sole means of generating electric power within the fuel cell. It is therefore desirable to reduce or eliminate any mixing of the reactants. Otherwise, a different result might occur, such as combustion, which does not produce electric power, and therefore reduces the efficiency of the fuel cell.
A typical fuel cell operates at a potential of less than about one (1) Volt. To achieve sufficient voltages for power generation applications, a number of individual fuel cells are integrated into a larger component. To create a fuel stack, an interconnecting member is used to connect the adjacent fuel cells together in an electrical series, in such a way that the fuel and oxidants of the adjacent cells do not mix together. For lower temperature fuel cells, e.g., those having an operating temperature of less than about 200° C., a large number of elastomer seals in compression may be used to separate the two reactants. However, elastomer seals cannot withstand the operating temperatures of high-temperature fuel cells. Consequently, other materials, such as glass ceramics, must be used to form the seals. Unfortunately, the seal performance of glass ceramics often remains problematic for high temperature fuel cells, due to its susceptibility to thermal shock and the induced brittleness and spillage. Moreover, the glass seals are not suitable for large gaps, and tend to have leakages over the large sealing perimeters.
Another type of seal used in planar SOFC's is the compression metal seal. These are internal manifolded seals, which are radically different from others, since bonding is not required. Here, sufficient compressive load is applied to deform the metal gaskets and therefore prevent gas leakage. However, in most situations, the internal manifolded seals used in planar SOFC systems typically share vertical loads between the seal region and the fragile ceramic cell or active area. Hence while applying the required load to get a gas-tight seal, there is a risk of breaking or cracking the active area of the cell. An attempt to decouple the load applied to the seal parts and active areas of the cells may prove to be a challenging task for design and manufacturing.
It would therefore be desirable to design a fuel cell stack wherein the cell load is substantially decoupled from the manifold loads. Elements of the present invention are directed at addressing these requirements.